Joints between surfaces of aircraft parts such as fasteners must be sealed to prevent fuel leakage. Aerospace sealants can be provided as two-component systems that must be mixed immediately prior to use or can be provided as pre-mixed one-component systems that are stored at low temperature, have a limited shelf live, and exhibit a typical working time of less than 48 hours. The sealants are typically applied by hand to pre-cleaned surfaces after which excess sealant must be removed. The process is time consuming, costly, and can result in variable seal integrity.
To address these issues, pre-applied sealant systems have been developed in which an unreacted moisture-curable sealant composition is applied to a surface of a part such as a fastener and a barrier coating is applied over the uncured moisture-curable sealant composition to prevent exposure to moisture and thereby prevent curing of the composition. The barrier coating prevents egress of moisture to the uncured composition and also serves to maintain the uncured viscous sealant composition in place. During installation of the part, the stress applied during assembly compromises or breaks the frangible barrier coating, causing the viscous sealant to flow between adjoining surfaces and exposes the uncured sealant composition to atmospheric moisture and thereby initiate sealant curing.
Moisture-curable sealant compositions have been developed that include silyl-blocked thiol-terminated polythioether and an epoxy curing agent. Moisture curable silyl-blocked thiol-terminated polysulfide and polythioether compositions are disclosed, for example, in U.S. Application Publication Nos. 2012/0121359 and 2012/0168055. When exposed to moisture, the silyl group hydrolyzes to provide a thiol-terminated prepolymer, which in the presence of an epoxy curing agent, crosslinks to form a cured sealant. In the self-sealing fasteners disclosed in these publications, the moisture-curable sealant composition is applied to a portion of the fastener to be sealed and a barrier coating is applied over the sealant composition to prevent moisture from reaching the sealant composition prior to assembly using the part.
Moisture-curable sulfur-containing prepolymer sealant compositions suitable for use in aerospace sealant applications are disclosed, for example, in U.S. Application Publication Nos. 2013/0345389, 2013/0344251, 2011/0009557, and 2013/0181161, and in U.S. application Ser. No. 14/200,687 filed on Mar. 7, 2014, each of which is incorporated by reference in its entirety.
Pre-applied moisture-curable sealant systems using alternative chemistries are desired.